1. Field of the Invention
This invention relates to a semiconductor integrated circuit and a video signal output circuit which are arranged to amplify and output a video signal.
2. Description of the Related Art
Conventionally, a video signal output circuit which amplifies and outputs a video signal, such as an NTSC (National Television Standards Committee) signal, is known. The video signal is made up of composite signals including a vertical synchronizing signal with a low frequency (for example, 60 Hz), a luminance signal with a frequency up to several MHz, etc. Video signal output circuits according to the related art have been improved to allow transmission of the video signal including these component signals without distortion of the waveform of the video signal.
FIG. 10 illustrates the composition of a video signal output circuit according to the related art. In the video signal output circuit 10 of FIG. 10, an operational amplifier 1 includes a non-inverted input terminal T1 and an inverted input terminal T2. A video signal is input to the non-inverted input terminal T1 of the operational amplifier 1, and the inverted input terminal T2 of the operation amplifier 1 is connected to an output terminal To of the operational amplifier 1. The video signal amplified by the operational amplifier 1 is output from the output terminal To. Moreover, a capacitor C1 which is provided for removing the direct-current component contained in the video signal output is connected to the output terminal To.
In the video signal output circuit 10 of FIG. 10, a capacitor with a large capacitance is used as the capacitor C1 in order to eliminate distortion of the waveform of the video signal output. However, if the capacitance of the capacitor C1 is increased, the capacitor C1 is increased in volume (size) and the mounting area of the capacitor C1 is increased. This is not appropriate for the video signal output circuit from a viewpoint of miniaturization of products.
FIG. 11 illustrates the composition of another video signal output circuit according to the related art. In the video signal output circuit 20 of FIG. 11, a circuit which is provided for compensating distortion (sag) of the waveform of a video signal is arranged so that the capacitance of the capacitor C1 is decreased.
In the video signal output circuit 20 of FIG. 11, the operational amplifier 1 is provided with a sag compensation terminal Ts in addition to the output terminal To. The sag compensation terminal Ts is connected to both the inverted input terminal T2 of the operational amplifier 1 and one end of a resistor R1. The other end of the resistor R1 is connected to the output terminal To of the operational amplifier 1.
In the video signal output circuit 20 of FIG. 11, a capacitor C2 is arranged between one end of the capacitor C1 and the sag compensation terminal Ts of the operational amplifier 1, and this capacitor C2 is connected in parallel with the capacitor C1. The video signal that is output from the operational amplifier 1 and passed through the capacitor C1 is returned back to the sag compensation terminal Ts.
In the video signal output circuit 20 of FIG. 11, the capacitance of the capacitor C1 can be reduced by this composition, and distortion of the waveform of the video signal output can be compensated (sag compensation). For example, Japanese Laid-Open Patent Publication No. 2004-274434 discloses a video signal output circuit which performs sag compensation for a video signal.
However, the use of a lowered supply voltage in recent years makes it difficult for the video signal output circuit to perform sag compensation. For example, if the supply voltage to the operational amplifier 1 as illustrated in FIG. 11 is changed to a lowered supply voltage, the dynamic range of the operational amplifier 1 will be insufficient for the amplitude of the video signal, which will make it difficult to perform sag compensation for the video signal effectively. As a result, a lack of a synchronizing signal in the video signal will take place and detecting a synchronizing signal in the video signal will be impossible.